1. Field of the Invention
The present invention relates to an aluminum vacuum brazing method and furnace for brazing a workpiece, carried into a vacuum brazing furnace, with a brazing filler material of aluminum alloy containing Mg, as well as an aluminum brazing sheet for vacuum brazing.
2. Description of the Related Art
Heretofore, based on the basic technique as disclosed in U.S. Pat. No. 3,321,828, there is known a method of vacuum brazing aluminum that uses a brazing sheet comprising a core member cladded with a brazing filler-metal containing Mg (magnesium) added thereto.
This addition of Mg to the brazing filler metal is made in expectation of (1) breakage of an oxide film on the brazing sheet surface, (2) removal of an oxidizing atmosphere (H.sub.2 O, O.sub.2) within the furnace by use of the function of gettering, and (3) reducing function.
However, Mg, which is necessary for aluminum vacuum brazing, is adsorbed on furnace walls and on mechanical parts to cause mechanical troubles. In addition, the brazing quality is reduced by water (H.sub.2 O) adsorbed on Mg which is adsorbed on walls.
In view of the above, Japanese Patent Unexamined Publication No. 63-52764 proposes a following technique. A workpiece is carried into a heating chamber covered by heat shielding Plates in the furnace. In order to prevent Mg vapor to scatter into the furnace, a gas flow port of the heating chamber is closed by a shielding member when the temperature in the furnace reaches some predetermined temperature at which Mg starts evaporation.
In a conventional aluminum vacuum-brazing method, the concentration of Mg in the brazing filler metal cladded over a brazing sheet was usually in a range of 1.2 wt. % to 2 wt. %. In view of decreasing the Mg adsorption on furnace walls and resource saving, it is desired to decrease the Mg concentration in the brazing filler metal.
Decreasing the Mg concentration, however, weakening the Mg function pointed before: (1) breakage of an oxide film. (2) gettering, and (3) reducing function.
It is apparently seemed from the above-cited Japanese Patent Laid-Open No. 63-52764 that densification of Mg vapor in the furnace can be expected and that the concentration of Mg in the brazing filler metal may be lowered. The above proposed technique, however, does not lessen the Mg content, as explained below.
Because the above proposed technique is specifically aimed to solve the Mg contamination problem of the furnace caused by evaporation and absorption of Mg, the gas flow port of the heating chamber is closed at about the furnace- (or the workpiece-)temperature that Mg starts evaporating. Accordingly, even though densification of Mg vapor in the heating chamber might be expected, the large amount of remaining water vapor in the heating chamber, unless evacuated sufficiently, disposes the brazing quality.
In other words, although densification of Mg vapor in the heating chamber has been achieved in the past, no measures have been considered in point how to decrease the Mg content.